1. Field of the Invention
This invention relates to a data-written medium having a variety of colors, whose data can be read with a light source having a main wavelength of 600 to 700 nm. More specifically, it relates to a bright-colored data-written medium whose data can be read with a light source having a main wavelength of 600 to 700 nm and which is functionally excellent in the prevention of falsification and forgery.
2. Description of Related Art
In recent years, control systems using bar codes such as physical distribution control systems, product control systems, material control systems, inventory control systems and POS systems have been developed and applied to various fields as a means of obviating complicated key inputting and bookkeeping. The procedures for marking bar codes are classified into a source marking procedure for producing a large number of identical bar codes and an in-store marking, in-house marking or on demand marking procedure for producing a limited amount of individual bar codes. The former procedure is carried out by an offset printing, gravure printing, flexographic press printing, relief printing, or screen printing method. The latter procedure is carried out by using a printer of a thermal transfer, thermal coloring or wire dot type, an ink-jet printer, a laser printer, or the like. Data processing is usually carried out as follows. Printed bar bode or character data is measured for its reflective strength by means of a light source such as a 632.8 nm He-Ne laser, a 660 nm light-emitting diode, a 670 nm visible light laser diode, a 690 nm light-emitting diode, a 780 nm laser diode, a 940 nm infrared-emitting diode, or the like, inputted as coded data or character data, and processed with a computer.
In general, the inks used to print these bar code and character data contain, as a dyestuff, a carbon black having an absorption in a wide range from a visible light region to a near infrared light region. And, due to bar code designing or in order to improve product evaluation, a study is made of the following inks or materials: dark (brown, dark blue, dark green, etc. ) chromatic color inks containing carbon black in combination, blue or green pigment-containing inks for detection with a 600 to 700 nm light source, thermal coloring paper using light-emission of a leuco dye, nigrosine type dye-containing inks for an ink-jet printer; and the like.
These inks or materials for any use are required to have sufficient absorption characteristics in the wavelength region of a light source to detect the data. In particular, the selection of light absorbers for a specific wavelength is essential to determine color indication on hue, saturation and brightness, detection sensitivity or weatherability of the data-written medium.
When near infrared absorbers are used, no absorption occurs in the visible light region, and therefore, even the combined use of these materials with a chromatic color dyestuff does not change the brightness, hue, saturation, etc., of the dyestuff itself. However, when an attempt is made to detect light absorption at a 600 to 700 nm (so-called visible light) wavelength region in order to read coded data or character data, it has been impossible to produce a data-written medium of any color other than blue or green, since the absorber itself has its own color.
On the other hand, in recent years, bar code readers increasingly use a 600 to 700 nm visible light wavelength region, since a shorter wavelength can be increasingly used for detection, and the sensitivity of a detection device has been increased with the development in the laser diode, the gas laser and the charge coupled device.
In view of the above problems, the present inventors have made a study to achieve the following characteristics of an absorber to be used in the 600 to 700 run visible light region.
1) The absorber is required to have sufficient absorption in the 600 to 700 nm wavelength region.
2) The absorber is required to exhibit lower absorption of light in a 400 to 600 wavelength region.
3) The absorber is required to have light resistance higher than that of conventional printing inks.
As a result, the present inventors have found that a data-written medium which can have a variety of colors usable with a conventional bar code reader or an optical character reader can be obtained by using 0.1 g/m.sup.2, in an ink layer, of a chromatic color dyestuff having substantial absorption at 600 to 700 nm in combination with at least one other dyestuff. That is, it has been found that when not more than a specified amount of the chromatic color dyestuff is applied in combination with at least one other warm color dyestuff, a clear-color data storage medium free from turbidity, e.g., a barcode, can be obtained.